1. Field of the Invention
This invention relates to a two-way cable television system. More particularly, the invention relates to a chargeable-program-receiving limit setting system in a two-way cable television system.
2. Background of the Invention
In general, in a cable television system (hereinafter referred to as "a CATV system") there are non-chargeable (or free) programs and chargeable programs. For chargeable program, the charging information for chargeable programs received by each subscriber is stored in a register in the terminal unit of the subscriber, and is then read out by polling at the central facility in the CATV system.
A conventional two-way CATV system of this type is as shown in FIG. 5.
In FIG. 5 a central facility 1 transmits data and video signals from a video source 6 and receives "up" data. A CPU (central processing unit) 7 performs data communication, audience control and charging process. An FSK transmitter 8 transmits data to CATV lines. A modem 9 receives "up" data through telephone lines from subscribers.
Further in FIG. 5, a CATV terminal unit 2 is provided to each subscriber. An FSK receiver 10 demodulates data which are transmitted from the central facility 1 (hereinafter referred to as "a center"). An A controller 11 performs the audience control of the terminal unit 2 according to the operation made by the subscriber or according to "down" data or performs data communication with an "up" communication module 3. The CATV terminal unit 2 further includes a converter 12 and a descrambler 13 and is connected to a television set 4. A telephone line exchange network 5 links the central facility 1 to the CATV terminal unit 2.
The arrangement of the "up" communication module 3 is as shown in FIG. 6. A telephone modem 14 applies "up" data to telephone lines which are to be transmitted to the center 1 from the terminal unit 2 and a B controller 15 performs communications with the A controller 11, controlling an event register 16 (described later), and transmitting "up" data. The event register 16 is used to store a history of received chargeable programs (hereinafter referred to as a "chargeable program reception history").
A charging system in the two-way CATV system thus organized will be described.
The A controller reads a chargeable program identifying code (hereinafter referred to as "an event code") contained in a video signal transmitted through the descrambler 13, and communicates it to the B controller 15 in the "up" communication module 3. The B controller 15 refers to the event register 16 to detect whether or not that event code has been stored therein. If not, the controller B causes the event register 16 to store the event code as a new one. In this manner, whenever the subscriber receives a chargeable program, the event code is stored in the event register 16. The event registers 16 of all the terminal units 2 at the subscribers are equal to one another in storage capacity. In each of the event registers, the chargeable program reception data can be stored up to the limit of the storage capacity.
When the event codes have been stored in the event register to its full storage capacity chargeable programs cannot be received any longer.
The chargeable program reception data thus stored in the event register 16 is periodically transmitted to the center 1. That is, when a chargeable program reception data collecting instruction is issued from the center by polling, the contents of the event register 16 at that time are read out by the B controller in the "up" communication module 3 and are modulated by the telephone modem 14. The output of the telephone modem 14 is transmitted as "up" data to the center 1 through the exchange network 5. At the center 1, the "up" data is demodulated by the telephone modem 9, and according to the output of the telephone modem 9, the charging process is carried out by the CPU 7.
In the case where the "up" data communication is performed through telephone lines, the time required for collecting "up" data from each terminal unit is of the order of thirty seconds. As the CATV system increases in scale, the time required for collecting "up" data from all of the terminal units is increased, for instance, to several days. If, during this period, the up communication module 3 is disassembled, and the event register 16 is modified to erase the event codes, then the subscriber will not be charged for the reception of the chargeable programs.
The monetary loss due to such dishonest reception of chargeable programs may be reduced by decreasing the storage capacity of the event register. However, the method is still disadvantageous in that the chance of earning an income is lost as much as the storage capacity of the event register is decreased.